The Telescope Lab
The purpose of this lab is for you to measure and investigate the optical properties of your telescope. In so doing you should gain a better understanding of the concepts involved.
Telescope, meter stick, ring stand, red clamp, index card (i.e. a “screen”)
q Determine the focal length of the objective and the ocular (eye piece). This is done by projecting the image of a very distant object onto a screen (index card). Secure the lens in question to the ring stand and align it with a distant object such as a tree out the door. Place the screen on the side of the lens opposite the distant object. Adjust the position of the lens and/or screen until a focused and clear image is formed. Use the meter stick to measure the distance from the center of the lens to the image on the screen. This distance is the focal length. Repeat for the other lens and record the results for both on the data table.
Part B – Magnification (Theoretical)
q Using the measured focal lengths determine the theoretical magnification of the telescope by dividing the focal length of the objective by the focal length of the ocular. Show the calculation that you make. Record the result on the data table.
Part C – Magnification (Measured)
q Now determine the magnification of the assembled telescope using a more direct method. Secure the telescope to the ring stand and focus on the large grid pattern. Look through the scope with one eye while keeping the other eye open. With a little patience you should be able to see both images at the same time – the image of the grid seen with the naked eye and the magnified image of the grid seen through the telescope. Adjust the telescope so that the corner of one of the magnified boxes overlaps the unmagnified grid at the origin. Then “measure” the magnified box’s width in terms of the unmagnified boxes. Or in other words, the magnification is how many times larger the box appears when magnified. This ratio of apparent size is the magnification of the telescope. Record the result in the table.
Part D – Field of View
q Secure the telescope to the ring stand and focus on the large grid pattern. Look through the scope and “measure” the width of your view through the scope by simply counting the number of squares across. Note that each square has a width of one inch. Use the distance to the pattern and the skinny triangle approximation to calculate the angular width of the view through the telescope. This is called the Field of View (FOV).
Part E – Resolution (Theoretical)
q Measure and record the diameter of your telescope’s aperture (this is the diameter of opening in the cardboard “washer”). Use this value to determine the theoretical diffraction limit on angular resolution for your telescope in units of arc seconds. Assume a wavelength of light equal to 600 nm. Show the calculation that you make. Record the result on the data table.
Part F – Resolution (Measured)
q Determine the angular resolution of the assembled telescope. Secure the telescope to the ring stand and focus on the pattern of lines. Record the minimum separation visible through the telescope and record the distance from the telescope to the pattern. Note: the telescope must be a sufficient distance so that not all of the pattern can be resolved – which is to say you must “push” the telescope to the limits of its resolution.
q Using the measurements from above, determine the angular resolution of the telescope in arc seconds. Show the calculations that you make. Record the result on the data table.
q Try projecting an image without the cardboard washer in place. Carefully remove the washer and replace the lens with the flat side facing outwards. Remove the eyepiece and replace it with a piece of paper held taut across the end of the tube by a rubber band in order to form a projection screen. Try observing a light bulb projected onto the screen. Hold the cardboard washer over the end and observe how this changes the image. Is there any effect on the brightness of the image? Is there any effect on the clarity or resolution of the image? Record your observations on the data table.
q Measure and record the diameter of the aperture with and without the washer and determine the ratio of the amount of light gathered without the washer compared to the light gathered with the washer in place. Show the calculations that you make. Record the result.
1. Write a brief paragraph comparing and contrasting the theoretical and measured values for your telescope’s magnification. Things to consider: Are the results the same? If not, which do you think is more accurate? Why?
2. Write a brief paragraph comparing and contrasting the theoretical limit on angular resolution with the measured value. Things to consider: Are the results the same? If not, why not? Which value is larger and why?
3. Briefly discuss the use of the cardboard washer. Discuss advantages and/or disadvantages to its use.
4. Briefly discuss how this lab exercise has helped or not helped your understanding of the telescope. Are there any suggestions for improvement? Better ways to obtain the data?

Focal Length 
Objective Lens 

Eyepiece Lens 

B. Magnification (Theoretical)
Calculation:
Magnification (Theoretical) 

C. Magnification (Measured)
Magnification (Measured) 

D. Field of View
Width of view through telescope 

Distance from pattern to telescope 

Calculation:
Field of View (Calculated) 

E. Resolution (Theoretical)
Calculation:
Resolution (Theoretical) 

F. Resolution (Measured)
Minimum separation of lines visible 

Distance from pattern to telescope 

Calculation:
Resolution (Measured) 

Observations:
Aperture diameter without washer 

Aperture diameter with washer 

Calculations:
Factor of light gathering increase without washer 
